1. Field of the Invention
The present invention relates to an apparatus and method for detecting a packet of an Orthogonal Frequency Division Multiplexing (OFDM) signal; and, more particularly, to an apparatus and method for detecting a packet of a zero-padded OFDM signal, which are capable of determining if a packet exists in a reception (RX) signal by comparing a cross-correlation value of an OFDM signal delayed by a predetermined sample time, e.g., a zero-padded sample time, with a power value of the RX signal, thereby increasing a packet detection probability and preventing a false alarm.
This work was supported by the IT R&D program of MIC/IITA [2006-S-071-02, “Development of UWB Solution for High Speed Multimedia Transmission”].
2. Description of Related Art
A conventional packet detector for an OFDM system separately calculates a cross-correlation value and a power value of an RX signal. The conventional packet detector determines whether a packet of the RX signal is detected or not by comparing a cross-correlation value of a preamble sample with a value obtained by multiplying the power value of the RX signal by a preset threshold value.
An operation of the conventional packet detector will be described below in detail.
The conventional packet detector for the OFDM system calculates cross-correlation values of N preamble samples by using a cross-correlation function, which is expressed as the following Equation 1.
                              Cor          ⁡                      (            n            )                          =                                                                        ∑                                  i                  =                  0                                                  N                  -                  1                                            ⁢                                                r                  ⁡                                      (                                          n                      -                      i                                        )                                                  ⁢                                                      p                    *                                    ⁡                                      (                                          N                      -                      1                      -                      i                                        )                                                                                            2                                    Eq        .                                  ⁢        1            
where Cor(n) is a cross-correlation value at an n-th sample time, r(n) is the RX signal received at the n-th sample time, and p*(j)(j=0, . . . , N−1) is a conjugate complex number of a j-th preamble sample previously stored in a packet detector of a receiver.
In Equation 1, |a|2=Re(a)2+Im(a)2 , where |a|2 is a value obtained by summing a square of a real part of the complex number “a” and a square of an imaginary part of the complex number “a”.
The conventional packet detector for the OFDM system calculates the power values of the RX signal, based on the following Equation 2. That is, the conventional packet detector sums the square of the real part and the square of the imaginary part of the RX signal received at each sample time and stores the resulting values in N buffers. Then, the conventional packet detector calculates the power values of the RX signal by summing the squares of the RX signal stored in the N buffers.
                              Pow          ⁡                      (            n            )                          =                              ∑                          i              =              0                                      N              -              1                                ⁢                                                                  r                ⁡                                  (                                      n                    -                    i                                    )                                                                    2                                              Eq        .                                  ⁢        2            
where Pow(n) is the power value at an n-th sample time, and r(n) is the RX signal received at the n-th sample time.
Then, the conventional packet detector multiplies the power value of the RX signal, which is calculated using Equation 2, by a preset threshold value (Threshold_value), as expressed in the following Equation 3.
The conventional packet detector determines if the cross-correlation value Cor(n) calculated using Equation 1 is greater than the value obtained by multiplying the power value Pow(n) calculated using Equation 2 by the threshold value Threshold_value, as expressed in the following Equation 3. The power related threshold value is applied to a packet detection process of the packet detector after an optimal value is selected through a simulation and test under a variety of channel environments.Cor(n)>Pow(n)×Threshold_value   Eq. 3
where Cor(n) is the cross-correlation value at the n-th sample time, Pow(n) is the power value of the RX signal received at the n-th sample time, and Threshold_value is the preset power related threshold value.
If the cross-correlation value Cor(n) is greater than the value obtained by multiplying the power value Pow(n) by the threshold value Threshold_value, the conventional packet detector determines the packet as being detected from the RX signal.
If the cross-correlation value Cor(n) is greater than the value obtained by multiplying the power value Pow(n) by the threshold value Threshold_value, the conventional packet detector determines the packet as being detected from the RX signal, transmits the determination value to a controller of a receiver, and terminates the packet detection process.
The conventional packet detector for the OFDM system has a problem in that a packet detection probability is reduced in a channel environment where multi-path is serious. This is because the cross-correlation value in the channel environment where the multi-path is serious becomes relatively smaller than the cross-correlation value in a channel environment where no multi-path exists. The packet detection process is performed by comparing the cross-correlation value with the value obtained by multiplying the power value Pow(n) by the power related threshold value Threshold_value. Thus, if the cross-correlation value becomes small, the packet detection probability will be reduced.
In addition, the conventional packet detector for the OFDM system has a problem in that a false alarm increases in a channel to which additive white Guassian Noise (AWGN) is applied. For example, when a signal-to-noise ratio (SNR) of an RX signal is 0 dB in an ultra wide-band (UWB) channel environment, a false alarm may occur at more than 10% of transmission frames and a packet detection may frequently fail.
Furthermore, in the case of the OFDM system that transmits the preamble pattern generated by repeating a code set with a predetermined length, if a cross-correlation value is calculated through the conventional packet detector, a peak value is obtained at a reference position, but a cross-correlation value with a predetermined magnitude is periodically obtained at positions other than the reference position. If the conventional packet detection method is applied with the power related threshold value set to a small value, the packet may be frequently detected at undesired positions other than the reference position.